Surgical stents have long been known which can be surgically implanted into a body lumen, such as an artery, to reinforce, support, repair or otherwise enhance the performance of the lumen. For instance, in cardiovascular surgery it is often desirable to place a stent in the coronary artery at a location where the artery is damaged or is susceptible to collapse. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery. One form of stent which is particularly desirable for implantation in arteries and other body lumens is a tubular stent which is formed as a complete tubular cylinder and can be radially expanded from a first smaller diameter to a second larger diameter. Such radially expandable stents can be inserted into the artery by being located on a catheter and fed internally through the arterial pathways of the patient until the unexpanded stent is located where desired. The catheter is fitted with a balloon or other expansion mechanism which exerts a radial pressure outward on the stent, causing the stent to expand radially to a larger diameter. Such expandable stents exhibit sufficient rigidity after being expanded that they will remain expanded after the catheter has been removed.
Radially expandable stents come in a variety of different configurations to provide optimal performance in various different particular circumstances. For instance, the patents to Lau (U.S. Pat. Nos. 5,514,154, 5,421,955, and 5,242,399), Baracci (U.S. Pat. No. 5,531,741), Gaterud (U.S. Pat. No. 5,522,882), Gianturco (U.S. Pat. Nos. 5,507,771 and 5,314,444), Termin (U.S. Pat. No. 5,496,277), Lane (U.S. Pat. No. 5,494,029), Maeda (U.S. Pat. No. 5,507,767), Marin (U.S. Pat. No. 5,443,477), Khosravi (U.S. Pat. No. 5,441,515), Jessen (U.S. Pat. No. 5,425,739), Hickle (U.S. Pat. No. 5,139,480), Schatz (U.S. Pat. No. 5,195,984), Fordenbacher (U.S. Pat. No. 5,549,662) and Wiktor (U.S. Pat. No. 5,133,732), each include some form of radially expandable stent for implantation into a body lumen.
Some problems which have been exhibited by prior art stents include that the inner and outer surfaces of the stents are not sufficiently streamlined or finely enough polished to prevent certain medical complications. For instance, thrombus, a phenomenon where a fibrous clot forms within cracks and other irregularities in the surface finish of an implanted object (such as a stent), is enhanced when the surfaces of the stent are not finely polished. Additionally, when the inner surface of the stent is substantially planar and has abrupt edges along borders thereof, turbulence is introduced into the blood. When a stent having such an abrupt edge is implanted into an artery, plaque and other deposits are provided with a site for collection and potential narrowing of the arteries and restriction of blood flow. This plaque buildup adjacent an implanted object (such as a stent) is referred to as "restenosis."
While many prior art stents do exhibit somewhat polished surfaces, they are typically not sufficiently finely polished, especially on tubular stents having smaller diameters, to prevent restenosis and thrombus adjacent the stent after the stent is implanted into the artery. Such prior art stents also lack a streamlined contour to minimize disruption of bodily fluid flow through the lumen and to further discourage restenosis surrounding the stent.
A primary reason why prior art stents fail to exhibit sufficiently finely polished surfaces to avoid the drawbacks discussed above is the lack of a polishing process which can effectively provide the finely polished surface desired, especially on stents having smaller inner diameters. Stents are typically polished in one of two processes, either chemical etching or electropolishing. With chemical etching, chemicals are used which react chemically with the material forming the stent, causing the material forming the stent to be driven into solution. Chemicals are selected which have a strength sufficient to cause rough areas of the stent to be dissolved, but not so strong that smooth areas of the stent are detrimentally altered. Chemical etching, while somewhat effective in removing gross irregularities from the surfaces of the stent, fail to adequately provide the desired finely polished surface.
Electropolishing typically involves providing an electrolytic solution, placing the stent within the electrolytic solution, placing a cathode within the solution and not contacting the stent and coupling an anode to the stent. When an electric voltage is provided between the anode and the cathode, the stent is caused to lose portions of its outer surface when the elements forming the stent are driven into solution and carried to the cathode for deposition upon the cathode. In essence, such electrolytic polishing is the reverse of commonly used electrical plating processes with material from the surface of the stent being removed rather than added to the stent. The rougher surfaces of the stent are more readily driven into solution and hence removed from the surfaces of the stent, smoothing the surfaces of the stent somewhat.
Because the surfaces of the stent forming the inner diameter of the stent benefit from a high degree of polishing, one known technique is to form the cathode as a thin wire passing along a central axis of the stent entirely through the stent from one end to the other, but without physically contacting the stent. When a voltage is provided between the cathode wire passing along the central axis of the stent and the stent itself, the inner surfaces of the stent are provided with the greatest electric field density and hence are the surfaces which are most polished during this process. While typically more effective than chemical etching, electrolytic polishing also fails to provide a sufficiently finely polished stent to significantly discourage thrombus and restenosis adjacent surfaces of the stent.
Accordingly, a need exists for a method and apparatus for polishing surfaces of a radially expandable surgical stent, and particularly the surfaces forming the inner diameter of the stent, with a sufficient degree of polish to reduce or eliminate the occurrence of thrombus and restenosis when surgical stents are implanted within a body lumen.